SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - NEW ACCOUNTING STANDARD (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Sep. 30, 2016
Assets
Other noncurrent assets	$ 62,356	$ 60,196
Total noncurrent assets	683,395	703,654
Total assets	3,877,926	3,718,570
Capitalization and Liabilities
Long-term debt	897,703	1,055,038
Total capitalization	2,182,304	2,221,629
Total capitalization and liabilities	$ 3,877,926	3,718,570
As Previously Reported
Assets
Other noncurrent assets		68,708
Total noncurrent assets		712,166
Total assets		3,727,082
Capitalization and Liabilities
Long-term debt		1,063,550
Total capitalization		2,230,141
Total capitalization and liabilities		3,727,082
Effect of Change
Assets
Other noncurrent assets		(8,512)
Total noncurrent assets		(8,512)
Total assets		(8,512)
Capitalization and Liabilities
Long-term debt		(8,512)
Total capitalization		(8,512)
Total capitalization and liabilities		$ (8,512)